1. Field of the Invention
The present invention relates to a non-volatile semiconductor memory device.
2. Description of Related Art
In a sense amplifier of a flash memory which is a non-volatile semiconductor memory device, as disclosed in Japanese Unexamined Patent Application Publication No. 2006-114154, for example, current from a target memory cell and current from a reference cell (reference current) are read out through the differential amplification. In this flash memory, a transistor having the same structure as the memory cell is used as a reference cell in order to reduce the influence of power supply voltage fluctuation.
However, according to such a structure, it is needed to set the condition to perform reading/writing from/to each reference cell and to generate reference current. This condition setting process requires a larger amount of time and cost.
Japanese Unexamined Patent Application Publication No. 60-167197 discloses the structure of constantly applying certain voltage to a gate of a MOS transistor which is a reference cell. According to such a structure, there is no need to perform the condition setting stated above. Instead, a switch or a reference cell selection transistor is required to switch a sense amplifier and a reference cell to a conduction state at a predetermined timing.
By the way, in the flash memory, a latch type sense amplifier has been used in various situations in accordance with increased speed of the reading operation, low power consumption, and low voltage operation. The reading operation is performed as follows when such a sense amplifier is applied to Japanese Unexamined Patent Application Publication No. 60-167197.
First, two sense terminals of the sense amplifier are precharged. Next, a read target memory cell and a reference cell selection transistor are activated and sampled at the same time. One sense terminal of the sense amplifier is made conductive to the read target memory cell, and the other sense terminal is made conductive to the reference cell. Hence, charge precharged in each sense terminal is discharged, and a potential difference is generated between both sense terminals. Then, the sense amplifier is activated, which amplifies the potential difference between the both sense terminals.